GTL (gas to liquids) is a technique for producing petroleum products such as naphtha, gas oil, kerosene, and the like from light hydrocarbon gas. A GTL plant includes, for example, a synthesis gas section for producing synthesis gas by reforming natural gas as the light hydrocarbon gas, a Fisher-Tropsch (FT) section for producing liquid hydrocarbon from the synthesis gas produced in the synthesis gas section by FT synthesis, and an upgrading section for performing hydroprocessing on the liquid hydrocarbon produced in the FT section to produce oil products such as naphtha, gas oil, kerosene, and the like.
In the synthesis gas section, the natural gas is added with hydrogen for desulfurization, steam and carbon dioxide (CO2) are mixed therewith, and the mixture is supplied to a synthesis gas reformer to be reformed, thereby producing synthesis gas mainly containing carbon monoxide (CO) gas and hydrogen (H2) gas. Here, temperature control of the synthesis gas of the outlet of the synthesis gas reformer influences a H2/CO ratio of the synthesis gas, and further influences the production fraction and purity of end products such as naphtha, gas oil, kerosene, and the like.
As a method of controlling the temperature of the synthesis gas of the outlet of the synthesis gas reformer, there is a conventional temperature control (TC)/pressure control (PC) cascade control method of controlling the output of a burner that is a heat source of the synthesis gas reformer according to the outlet temperature of the synthesis gas reformer (for example, see Non-Patent Document 1). The control method is described with reference to FIGS. 9 and 10. FIG. 9 is a view for explaining a temperature control system of a furnace 900. FIG. 10 is a flowchart for explaining a cascade control logic of the conventional method.
As illustrated in FIG. 9, the furnace 900 has a burner 902 and a heating pipe 904. The outlet of the furnace 900 is provided with temperature measuring device 922 for measuring the outlet temperature and temperature controller 924. In addition, on the inlet side (fuel gas supply side) of the burner 902, pressure measuring device 934 and a pressure control valve 940 are provided, and the pressure measuring device 934 and the pressure control valve 940 are connected to pressure controller 932.
Next, the temperature control method of the outlet of the furnace 900 is described. A to-be-heated fluid 910 is heated by the burner 902 while flowing through the heating pipe 904 and becomes a heated fluid 918. The temperature of the heated fluid 918 is measured by the temperature measuring device 922 provided on the outlet side of the furnace 900, and the opening degree of the control valve 940 is adjusted on the basis of the measured temperature. Accordingly, the pressure and flow rate of fuel gas 916 is controlled, and output control of the burner 902 is performed, thereby performing the temperature control of the heated fluid 918.
The above-mentioned TC/PC cascade control is described in detail with reference to FIG. 10. A target value (SV) of the outlet temperature of the furnace 900 is set in step S960. The temperature of the heated fluid 918 is measured by the temperature measuring device 922 thereby measuring a measured value (PV) in step S962. A temperature difference ΔT between the SV and the PV of the outlet temperature is calculated by the temperature controller 924 in step S964, and in order to compensate for the ΔT, control output of the furnace outlet temperature is performed in step S966. Next, in the pressure controller 932, a control target value (SV) of the pressure of the fuel gas 916 is set in step S968. The pressure measuring device 934 measures the pressure of the fuel gas 916 thereby measuring a measured value in step S970. A pressure difference ΔP between the SV and the PV of the pressure of the fuel gas 916 is calculated in step S972, and for the pressure control valve 940, control output for determining the opening degree of the pressure control valve 940 is performed in step S974 to control the output of the burner, thereby enabling control of the outlet temperature of the furnace 900.    [Non-Patent Document 1]: Instrumentation handbook, Instrumentation & Process Control Engineer's Association, May 1, 1991, p. 3-29.